Hydrothermal synthesis, structural characterization, and physical properties of a new mixed valence iron phosphate, SrFe3(PO4)(3)

A new mixed valence Fe(II/III) phosphate, SrFe3(PO4)(3), has been synthesiz ed hydrothermally and structurally characterized by single-crystal X-ray di ffraction, EDAX, magnetic susceptibility, Mossbauer, infrared and diffuse r eflectance spectroscopies, DTA, and bond valence sum calculations. The comp ound crystallizes in the orthorhombic space group mma (No, 74) with a = 10. 452(3), b = 13.429(3), c = 6.528(2) Angstrom, V = 916.3(4) Angstrom(3) and Z=4 with R/R-w = 0.0207/0.0307. The structure consists of a complex low-dim ensional framework constructed of FeO6 octahedra and PO4 tetrahedra that fo rm 2D channels in which the strontium ions are located. The framework conta ins two simple building blocks: rigid columns of edge-sharing Fe2P2O14 unit s and zigzag chains of alternating PO4-FeO6-PO4 units, both of which run pa rallel to the b direction. This linkage forms intersecting channels which r un parallel to the a and b axes. The divalent strontium cations reside with in these channels and create openings in the ab plane with dimensions of 6. 903(4) and 7.469(4) Angstrom, respectively. These openings can easily accom modate both small and large monovalent or divalent cations, as demonstrated by several cation exchange reactions. The Mossbauer spectral parameters ar e consistent with the structure and reveal the presence of mixed valent oct ahedral iron(II) and iron(III) sites in the expected two to one ratio. The quadrupole splittings observed for both iron sites are very large as is exp ected for the very asymmetric coordination environments found for each iron site. (C) 1999 Academic Press.